The error you mention happens because the data you return must have the same name as the "arg" you define below. On your code, "results" it's just raw data that doesn't has a name. Loopback's looking for something like:

results = ["foo, bar, baz"];

That's one thing. Another thing is that if you want random results (as one might think by the method's name), you need to provide the sample function, otherwise you're only returning all your data in the conventional (descending?) order and if the request it's big enough you might have issues.